Synapsid Lecture 5

Synapsids
• Main group we’re focusing on –mammals.
• Mammalia –monophyletic group.
• “Pelycosaurs” –non mammalian synapsids
• Three groups:
o Pelycosaurs (refer more to a grade than a clade) –primitive synapsids
o Therapsids –advanced synapsids
§ Mammals
• Monotremata
• Theria:
o Eutheria (Placentals)
o Metatheria (marsupials)
Mammals
• Diagnose them by soft-tissue characters:
o Mammary glands –important in reproductive mode (where mammals
get their name). Functional in the females (with exceptions).
o Hair
o Muscular diaphragm –partition that separates the thoracic from the
abdominal pelvic cavity
§ Muscle that is helpful in ventilating the lungs.
o Problematic if we go into fossil records, as they do not fossilize.
o What we tend to do is to look for other features of the skeleton that
help us define what mammals are and they survey these characters in
the fossil record.
§ Presence of 1 bone in each half of the lower jaw –the dentary.
• Upper joint: Squamosal, lower jaw: dentary
• Differs from reptiles:
o Upper joint: Quadrate, lower joint: articular.
(The quadrate-articular articulation)
§ Middle ear ossicles (little bones)
• Reptiles: 1 bone –the stapes (collamila in mammals).
• Mammals: 3 bones –the malleus, incus and
stapes/collamia (outside to inside, lateral-medial).
o Stapes is the same as the reptile; others are
derived from the other bones used to make up
the jaw joint in reptile. Seen as a transitional
sequence in embryos.
o The angular bone in the reptile becomes a part of
the bones forming the mammalian outer ear that
holds the eardrum –acultonpact.
§ These conditions are for modern mammalians, if we go back in
time there are several synapsids that approach the mammalian
form (in term of the skeletal organization), some became
extinct and some survived. We also see transitional forms in
which the upper jaw was formed by part of the squamosal and
part of the quadrate, while similarly the lower jaw was formed

Two main types:
by the dentary and articular bones. This is similarly observed
in embryos.
§ These early mammals –or synapsids do go back a long way.
The synapsids are one of the main groups of the amniotes –and
are quite old. Nearly wiped out during dinosaur ear (Triassic,
Jurassic and Cretaceous) and regained diversity in the Tertiary.
§ Pelycosaurs –dimetrodon (carnivores), edaphosaurus
(herbivores). Early synapsids. Limbs spread out to the side,
sprawling posture.
§ Therapsids –moschops, titanophoneus. Change limbs are
swung under the body, like a cat and moving on a parasagittal
plane. Increased speed (less lower body friction) opened up
different niches and is able to chase bigger and faster prey.
• Monotremata –platypus, echidna
o Shelled egg, but have hair and mammary glands.
o They still have a cloaca (literally “one opening”, for digestive and
reproductive tract).
o Lateral braincase walls is formed by a bone called periodic
• Theria:
o Lateral braincase walls formed by a bone called alisphenoid (distinct
from monotremes)
o Metatheria: pouched mammals or the marsupials, and several
extinct forms (fossil forms). Not all marsupials are pouched animals,
i.e. opossum (carnivorous) and kangaroos (deprotodons –enlarged
front teeth [incisors]). The young are born at a very immature, and
must make their way up to the pouch clamp on with their teeth and
finish development in the pouch.
o Eutheria: placental (placenta is also present in marsupials). Different
kind of a placental –their placental allows the young to stay in the
utero for a longer period of time, thus the young emerges at a later
time in development.
§ Main groups:
• Chiroptera: flying mammal, bats –one of the most
diversified and specious
• Rodentia: rodents, mice, rats –even more diversified.
Enlargement of the incisors, can set jaw in 2 position
grinding and gnarling.
• Carnivora:
o Cat-like forms
o Dog-like forms
• Ungulate: walking on their nail/hoofs (cows, giraffe
etc.)
o Artiodactyla: Grass, foliage. Cows, giraffes and
hippos. Parasonic symmetry, lines of symmetry
in fingers 3 and 4. Raise foot and walk on tip-toes.

§ Cetacea (whales and propoises) closely
related to hippos.
o Perissodactyla: Horse, Zebras and Rhinos.
Bizonic only the middle finger, horses walking on
their middle finger.
• Proboscidea: mammoths, elephants
• Sirenia: sea cows (gentle underwater cows).
• Zenapora: tree sloths, south American anteaters,
armadillos. Great rich fossil record.
• Primates: humans, and other great apes (gorillas,
orangutans).
o No particular advantage in reproductive strategy (despite the greater
quantity of placental vs. marsupials), but it seems that marsupials do
particularly well in response to stressful changes in the environment.
Whereas in stable environments, placental do well but when things
change so do their reproductive fortunes. In one you can ditch
(marsupials) the offspring when in stress, they may have multiple
offspring at the same time.